Recently the Discovery Channel had a show on that featured a Tricked Out Prius. A West Coast shop modified a Prius for a Toyota dealership in L.A.. One thing they did was to put 18 inch wheels on the thing. It looked neat, but can this be done without affecting calculation of mpg and modifying one of the ECU's?
Good luck finding a tire in that size, or an 18" rim narrow enough to fit a 185 width tire. Best fit in an 18" tire that is seems to be available is 205/40/18. Difference from OEM in rolling diameter is minimal.